1. Field of the Invention
The invention in general relates to electronic systems having a power failure monitoring function and more particularly to such a system that in a single parameter takes into account all the significant elements that relate to power failure monitoring.
2. Statement of the Problem
It is well-known that some electronic systems, particularly digital electronic systems such as computers, may be adversely affected by loss of power during operation. The adverse affects may include loss of important data from volatile storage and the introduction of errors. Thus such electronic systems often include a power monitor that signals the system when power loss is impending, allowing the system to accomplish certain housekeeping tasks and store critical information in non-volatile RAM, for example. In order for these tasks to be accomplished before power failure, such systems must include some power storage means that allows the power to hold up for a time before it fails. Generally, in digital computer systems that utilize external AC power which is converted to internal DC power by a suitable power supply, the monitoring means is included in the power supply and provides at least 4 milliseconds warning between the time the AC power fails and the time the DC output drops below a value where the adverse consequences occur. In such systems sufficient power is stored in the power supply filter capacitors to maintain the voltage above the critical value for the critical time. See for example U.S. Pat. No. 4,985,804 issued to W. Carroll Campbell et al. which discloses a monitoring means that senses when the AC input voltage drops below a threshold voltage. U.S. Pat. No. 4,658,204 issued to Peter A. Goodwin discloses that some prior art devices monitor the load voltage rather than the AC input voltage. This patent also discloses monitoring for a saturated state in the feedback control circuit in a regulated power supply. A problem with such a simple threshold voltage or saturation approach is that the monitor may detect short outages of power and signal the computer that power failure is imminent, which disrupts the operation of the computer, when, in actuality, the power supply would have been capable of maintaining operation of the computer with no disruption during the outage.
The computer system disclosed in U.S. Pat. No. 4,985,804 referenced above overcomes the above problem to some extent by signaling impending power failure only when the power drops below the threshold voltage for a certain number of half-cycles, which number of cycles can be varied manually, depending on the anticipated load of the system, by use of a potentiometer. Thus this prior art system allows matching of the monitoring to the load, which is a significant factor in determining how long the DC power will last after AC power is lost. However, there still remain problems with this approach. The first is that knowledge of the load is required, which may vary greatly for any given computer configuration. Thus one is forced to assume a worst-case condition of full load on the power supply. The second problem is that the hold-up time decreases significantly as the AC line voltage just prior to power loss is lowered, and thus one is forced to assume a low AC line condition. Applying both these conditions results in greatly reduced ability to coast through power outages.
Another approach to providing assured sufficient warning of impending power failure with ability to coast through short power outages is disclosed in U.S. Pat. No. 4,742,424 issued to Jeffrey A. Kautzer et al. This system uses a charge/discharge model that upon AC power failure estimates the time left before the power falls below an acceptable level. The disclosure indicates that the load may be estimated by sensing on/off conditions of system elements or measuring the current provided by the DC power supply, and using this to change the rate of count in a counter in the charge/discharge model or by inhibiting one or more logic elements for changing division ratios in the counter, though it does not go into details of how this would be implemented. While this charge/discharge model appears to take into account the significant parameters that relate to the energy in the system, i.e. the voltage, the energy storing capacity, and the load, the device disclosed is relatively complex and at best can only give an estimate of the time left before decay based on a model that only approximates the computer system. It would be very useful to have a system that takes into account the actual values of the voltage, current and energy storage ability, e.g. capacitance, of the system prior to the initiation of the warning signal, and does this using a single, easily measurable parameter of the electronic system.
Another aspect of the timely warning versus coast through problem is that different electronic systems generally require differing amounts of time to perform the operations necessary prior to power outage. As indicated above, U.S. Pat. No. 4,985,804 provides some solution to this problem in that the number of half-cycles over which the voltage may remain below the threshold, and thus the warning time can be adjusted by a potentiometer. Similarly, U. S. Pat. No. 4,742,424 discloses a plurality of threshold detector circuits that provide different lengths of warnings. In the former case the adjustment must be made manually, and in the latter case, each separate time requires a separate threshold detector circuit, which is relatively expensive. Both solutions offer only a limited number of different warning times. U.S. Pat. Nos. 4,122,359 issued to Ivars P. Breikas, 4,381,457 issued to James P. Wiles, and 4,818,891 issued to Don L. Drinkwater, disclose other solutions to the problem of providing coast-through ability for almost any system. These disclose a backup battery, an auxiliary power supply, and a power booster to permit the system to make it through loss of power. Other devices have shown the use of a capacitor for the same purpose. However, all these solutions are relatively complex and expensive. It would be highly useful to have a electronic system in which any desired length of warning could be easily provided.
Providing a display of power consumed by an electrical system is and old and crowded art. See for example U.S. Pat. No. 4,415,853 issued to Berish M. Fisher. However, up until now, no one has provided such a display on a computer, and no one has shown a simple and inexpensive way of providing such a display in a computer.
3. Solution to the problem
The present invention provides an electronic system having a power failure monitoring function that, like some of the simplest of the prior art power failure monitoring devices, uses a threshold of a single parameter of the system to trigger the power failure warning signal, however, unlike prior art systems, the threshold is a variable one.
The present invention provides an electronic system having a power failure monitoring function employing a variable threshold which can easily be calculated on the fly (in real time) and takes into account the significant properties of the system that effect the length of time that the energy in the system will sustain the functions necessary for orderly power down.
The invention further provides such a system in which actual, current values of the significant properties are used in calculating the threshold, and thus the threshold accurately reflects the state of the system at the time of power loss.
The invention also provides such a system in which the desired time prior to the power falling below an acceptable level is also factored into the threshold.
The invention thus provides a method and apparatus which optimizes the ability of a computer to ride through power outages without risking premature loss of power.
In addition, the invention provides a computer in which the actual system load power, which is calculated and used in determining the variable threshold, is displayed on the front panel of the computer. This information has been found to be highly useful in determining, for example, whether more computer cards can be added into the backplane without overloading the power supply, determining power source requirements, and determining cooling requirements.